In hydraulically operated power steering systems, a pump driven by the vehicle engine is provided for supplying fluid under pressure to a steering valve controlled by the vehicle steering wheel. In equipment such as earth movers, if the engine fails while the vehicle is in motion, the vehicle may be nearly impossible to steer especially if it is on a hill at the time of power failure.
The prior art discloses various arrangements whereby a supplementary or emergency pump is provided for supplying hydraulic fluid to the steering valve should the primary source of power fail. One example of such a system is disclosed in U.S. Pat. No. 3,631,937. In the system disclosed in that patent, pressurized fluid is provided by means of a supplementary pump which is driven by a drive connection with the wheels of the vehicle so that the supplementary pump is driven whenever the vehicle is in motion. The flow from this supplementary pump is added to the flow from the engine driven pump when the engine is in operation so that the fluid supplied to the steering valve is additive when both pumps are in operation. A desirable feature of the U.S. Pat. No. 3,631,937 is that the supplementary pump is reversably operated so that it will continue to supply fluid to the steering circuit regardless of the direction of movement of the vehicle.
Another approach to the problem is disclosed in U.S. Pat. No. 3,851,721. The system disclosed in this patent also includes an engine driven pump and a wheel driven pump. The fluid supplied to the steering circuit is the sum of the volumes discharged by the wheel and engine driven pumps. In contrast to U.S. Pat. No. 3,631,937, U.S. Pat. No. 3,851,721 does not supply fluid to the steering circuit when the vehicle is in reverse.
Although the systems described in the above two identified patents are simple and effective for many applications, a problem can arise in their use when both pumps are in operation, because the flow derived from the pumps is additive and may overwhelm or exceed the capacity of the steering circuit at high engine and ground speeds. Moreover there is a well recognized speed limitation for the wheel driven pump, since at high pump speeds, as is known to those in the art, gasification of the hydraulic fluid and cavitation at the inlet of that pump may result. A partial solution to this problem lies in the employment of a relatively small displacement wheel driven pump so as to prevent excess flow to the steering circuit when both pumps are in operation. However this approach limits the amount of fluid available from the wheel driven pump, so that steering may still be difficult upon engine failure. In addition, cavitation problems are not necessarily avoided by use of a small pump.
Other systems known in the art incorporate the use of a pilot valve which unloads the fluid discharged by the wheel driven pump to tank so long as the engine driven pump is in operation. Such systems do not solve the cavitation problem but do avoid the problem of overwhelming the steering circuit when both pumps are in operation and make it practical to use a wheel driven pump of large capacity if desired. However, the use of, pilot operated unloading valve introduces an additional complicating factor in the circuit. The unloading valve must be fool-proof in operation since the essence of an emergency system is that it must be absolutely dependable. Furthermore the use of an unloading valve results in momentary loss of steering in an emergency situation since the engine must stop before flow from the wheel driven pump is diverted to the steering circuit. Even momentary loss of control of the vehicle can be hazardous in a true emergency situation.